In the making of hollow plastic containers it is common to first form a preform or parison by injection molding and thereafter blow the preform within the confines of a mold to the shape of the container. When an orientable plastic is used the preform can be cooled and stored until needed and then reheated to an orientation temperature and blown to the final configuration in the mold.
Alternatively, the hot injection molded preform can be cooled to orientation temperature and immediately moved to a blow molding station and blown to induce orientation.
In one common type of preform or parison, which is injection molded, the preform includes a finish which has an external thread and defines the open end of the container that receives a closure, a radially outwardly extending annular ledge below the finish, a neck below the ledge, a shoulder, a generally cylindrical body and a closed bottom end.
When the resultant container is utilized to hold liquids that are filled at elevated temperatures or where the container and its contents are to be pasteurized, there may be a tendency for the finish portion to become distorted resulting in a poor seal with the closure that is applied to the finish of the container.
It has heretofore been suggested that the preform be made of two materials, the finish portion being made of polycarbonate and the remainder of the preform being made of the plastic material having the desired properties for the remainder of the container such as polyethylene terephthalate (PET). The preform is constructed so that the main PET body portion extends within the polycarbonate finish and over the free end of the finish. This has been achieved by injection molding the finish in a partible neck mold, moving the neck mold into registry with a mold which has a core and a cavity for forming the remainder of the main body and injection molding the plastic such as PET so that the finish and main body are bonded to one another.
Such an arrangement is helpful in defining more stable threads on the finish but is still subject to the deficiency of having a portion of the PET material covering the free end of the finish.
Another problem with such a method is that the neck mold which forms the finish must necessarily be used in forming the main body onto the finish.
Accordingly, among the objectives of the present invention are to provide a preform that is made of two plastic materials which are compatible with one another; wherein the plastic utilized for the finish has greater dimensional stability and is utilized to a minimum due to potentially higher cost; wherein less expensive material may be used in the main body of the preform; wherein the amount of more expensive finish material is minimized; wherein the resultant container has greater dimensional stability at the juncture of the finish and the main body portion and at the upper rim of the finish where filling machinery and the closure contact the finish portion.
In accordance with the invention, the preform for making plastic containers wherein the preform is first formed and thereafter blow molded immediately or cooled for storage, reheated to an orientation temperature range and blown to final container shape comprises an open ended finish part made of a first plastic material having threads on the outer surface of the open end and an elongated main hollow body of orientable strain-hardenable thermoplastic material molded and bonded in situ on said open ended finish. The hollow body includes a main body container forming portion and a bottom container forming portion closing the open end of the main body forming portion. The main body has a portion projecting axially within the open end of the finish at the end opposite the threaded end and terminating in spaced relationship to the open free thread bearing end of the finish and interlocked with the finish.